The present invention relates to a cordless telephone apparatus.
A cordless telephone apparatus composed of a base unit (base unit) connected to a telephone line and one or more handset units (handsets) for communicating with the base unit through radio waves has been widespread. In the United States, for example, ten channels (10 CH) have been assigned to the cordless telephone apparatus in a 46 MHz band and a 49 MHz band. The FCC standard defines in terms of these ten channels that it is not necessary to determine whether an electric field is present on a channel selected from the ten channels to be used (whether any signal is being received on the channel) before starting transmission of speech or control data on that channel, and therefore the transmission of speech or control data can be started on that channel even if an electric field is detected (for example, even if another cordless telephone apparatus is using the same channel).
Conventionally, for completing a connection between a base unit and a handset unit through a radio channel (hereinafter called the "radio link") in a reduced time, the cordless telephone apparatus manufactured based on the above-mentioned standard establishes the radio link in the following method.
When the base unit or the handset unit starts transmission, the transmission side utilizes the last used channel through which the previous radio link was formed between the base unit and the handset unit for communication, and control data is communicated therebetween through this channel to form the radio link. While the base unit and the handset unit subsequently proceed to a speech state, the base unit performs an electric field detection on the channel before entering the speech state. If no problems are found in terms of the control channel in this detection, the same channel as the control channel is used as a channel for speech communications (hereinafter called the "speech channel") through which the base unit and the handset unit enter the speech state.
However, if the electric field detection reveals that the channel is being used by another radio device with a high possibility (hereinafter a channel in use is referred to as a channel presenting a strong electric field), the base unit examines whether there are other empty channels (hereinafter, an empty channel is referred to as a channel presenting a weak electric field), selects one if empty channels are found, and determines that the selected channel is used as the speech channel. Then, the base unit transmits to the handset unit information indicative of the speech channel thus selected, a transmission frequency and a reception frequency are set to the frequency of the selected channel for both the base unit and the handset unit, and thus a speech communication is enabled through this channel.
As described above, conventionally, the base unit performs the electric field detection on a channel for proceeding to the speech state. If the channel presents a strong electric field, the base unit selects a speech channel from other channels which can be confirmed to be empty, so that the speech state may be occasionally entered through a different speech channel from the control channel through which control data was initially transmitted. Then, when another call is to be made after this speech communication has been ended, the channel last used for establishing the previous radio link between the base unit and the handset unit is used as a control channel through which control data is communicated therebetween, similarly to the above.
The FCC standard is to be revised after August 1995, and 15 channels are newly added to the conventional ten channels, so that a total of 25 channels are available for communications with the cordless telephone apparatus. Among these 25 channels, the conventional ten channels are newly assigned channel numbers 16CH-25CH without any change in frequency. The newly added 15 channels are assigned a lower frequency band than the conventional frequency band and new channel numbers 1CH-15CH, respectively.
As before, the transmission of speech or control data is permitted to start through one of the conventional ten channels, i.e., 16CH-25CH irrespective of its electric field state (the conventional ten channels are hereinafter referred to as the "channel group 1"). However, the revised standard defines that the newly added channels 1CH-15CH belong to a group of channels through which the transmission of speech or control data is permitted to start only after confirming that a weak electric field is present on the channels (hereinafter referred to as the "channel group 2").
Thus, the 25 channels are divided into:
Channel Group 1: 16CH-25CH PA0 Channel Group 2: 1CH-15CH
If the standard is revised as described above, the channels assigned to the cordless telephone apparatus will partially include a group of channels through which transmission is not permitted depending on their electric field states (channel group 2). When a call is made in the conventional method, the last channel used in the previous radio link, which will serve as a control channel in the next call, can be a channel belonging to the channel group 2. Thus, even if this channel is to be used as the control channel, it is not available when a strong electric field is detected on that channel. In this case, it is necessary to immediately look for a channel presenting a weak electric field among other channels, whereby a radio link may be occasionally formed with a large delay. This causes a problem that a cordless telephone apparatus cannot be used immediately at the time it is required.